pgsql: Suppress compiler warning in relptr_store().

Suppress compiler warning in relptr_store().

clang 13 with -Wextra warns that "performing pointer subtraction with
a null pointer has undefined behavior" in the places where freepage.c
tries to set a relptr variable to constant NULL. This appears to be
a compiler bug, but it's unlikely to get fixed instantly. Fortunately,
we can work around it by introducing an inline support function, which
seems like a good change anyway because it removes the macro's existing
double-evaluation hazard.

Backpatch to v10 where this code was introduced.

Patch by me, based on an idea of Andres Freund's.

Discussion: https://postgr.es/m/48826.1648310694@sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4fbbea2c1934092581662553fd4be9e84630fc5c

Modified Files
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src/include/utils/relptr.h | 17 +++++++++++++++--
1 file changed, 15 insertions(+), 2 deletions(-)